1. Field of the Invention
The present invention relates to an objective lens drive apparatus installed in an optical disk system and particularly, to a drive apparatus having objective lenses with different numerical apertures, arranged for switching from one to the other depending on the type of a recording medium to be scanned, and having an initialization function wherein one of the objective lenses is set to a predetermined location at the beginning of operation.
2. Description of the Related Art
Such objective lens drives apparatuses have been developed for use with playback systems for scanning a variety of advanced optical data recording mediums including an optical disk and an opto-magnetic disk. Some of the objective lens drive apparatuses are now available in the market for driving compact disks and CD-ROM disks.
Some types of objective lens drives provided for not only playback but also recording, particularly, with a magnet-optical or phase-change method. Most of the recording types are standardized for specific application. However, recently super-density types of the optical disk such as DVD format for maximizing the recording density have been developed and introduced. The optical disk of such a super-density type has pits of record unit formed much smaller in size than those of the common CD type to be scanned at a higher accuracy. Also, the optical disk of such a type has a substrate with a different thickness from that of the CD type. In addition, a playback system for the optical disk of such a super-density type employs a laser beam of a shorter wavelength to scan the pits, and an objective lens of a larger numerical aperture to focus a smaller spot of the laser beam on the optical disk.
As a variety of optical disks are employed, the drive unit may encounter a difficulty to scan each of the disks at a constant accuracy. This will require a user to have two or more drive units corresponding to the different types of recording medium to be scanned.
For overcoming the above drawback, a method is proposed as disclosed in U.S. Pat. No. 5,235,581 where a plurality of optical heads with different focal lengths are provided in one single optical disk drive. More specifically, the disk drive of the type has two optical heads arranged for performing tracking actions independently from each other thus allowing the recording and playing back of conventional optical disks such as CD as well as of super-density type.
However, the two optical heads are located opposite to each other about the center of the optical disk and therefore are not close to each other. Accordingly, when the optical disk of a cartridge (or casing) type which has a window for scanning (such as a CD-ROM or MO disk) is selected, the conventional optical disk drive does not allow either of two objective lenses to be located beneath the aperture of the window which has a substantially limited size. Furthermore, the optical disk drives of any type should be low in cost and the installation of such two optical heads will be expensive.
It is thus desired to provide an improved optical head which includes two or more different objective lenses arranged for switching from one to another. More preferably, such an improved optical head is capable of being initialized wherein one of the objective lenses is selected and presented at the beginning of operation. Furthermore, such an improved optical head is preferably adapted for identifying the type of a recording medium or optical disk to be scanned with the selected objective lens and thus allowing the selection of a corresponding type of the objective lens for the optical disk.